The present invention relates to a polycrystalline thin film having a pyrochlore type crystalline structure with well-aligned crystal orientation and a method of producing the same, and an oxide superconductor element of excellent superconducting property comprising an oxide superconducting layer formed on the polycrystalline thin film which has a pyrochlore type crystalline structure with well-aligned crystal orientation, and a method of producing the same.
The oxide superconducting materials which have been discovered in recent years are good superconducting materials that have critical temperatures above the temperature of liquid nitrogen However, there remain various problems to be solved before the oxide superconducting materials can be used as practical superconductors. One of the problems is the low critical current densities of the oxide superconducting materials.
The problem that the critical current density of the oxide superconducting material is low stems mainly from the electrical anisotropy which is intrinsic to the crystal of the oxide superconducting material. It is known that electric conductivity of the oxide superconducting material is high in the a-axis and b-axis directions of the crystal, but is low in the c-axis direction. Thus, in order to use an oxide superconducting layer formed on a substrate as a superconductor element, it is necessary to form an oxide superconducting layer of good crystal orientation on the substrate and to align the a-axis or b-axis of the crystal of the oxide superconducting material to the intended direction of current flow, while aligning the c-axis of the oxide superconducting material to the other direction.
Accordingly, a practice has been employed such that an intermediate layer having good crystal orientation made of MgO, SrTiO3 or the like is formed on a substrate such as a metal tape by means of a sputtering apparatus, and an oxide superconducting layer is formed on the intermediate layer. However, the oxide superconducting layer formed on an intermediate layer of this type by a sputtering apparatus has a critical current density (typically about 1000 to 10000 A/cm2) which is far lower than that of the oxide superconducting layer (typically several hundred thousands of A/cm2) which is formed on a single crystal substrate made of such a material. The cause of this problem is supposedly as follows.
FIG. 16 is a sectional view of an oxide superconductor element made by forming an intermediate layer 2 on a substrate 1 made of a polycrystalline material in the form of a metal tape or the like by means of a sputtering apparatus, and then forming an oxide superconducting layer 3 on the intermediate layer 2 by the sputtering apparatus. In the structure shown in FIG. 16, the oxide superconducting layer 3 is in a polycrystalline state in which a multitude of crystal grains 4 are bonded together in a random manner. These crystal grains 4 individually show the c-axis of crystal being oriented somewhat perpendicularly to the substrate surface, but the a-axis and b-axis are randomly oriented.
When the a-axis and b-axis are randomly oriented among the crystal grains of the oxide superconducting layer, degradation in the superconducting properties, particularly in the critical current density, would be caused due to quantum coupling of the superconducting state being lost in the grain boundaries in which crystal orientation is disturbed.
The cause of the oxide superconductor element turning into a polycrystalline state with the a-axis and b-axis randomly oriented is supposedly as follows: since the intermediate layer 2 formed below the oxide superconducting layer is polycrystalline where the a-axis and b-axis are randomly oriented, the oxide superconducting layer 3 would be grown in such a condition as to match the crystal structure of the intermediate layer 2.
The present inventors found that an oxide superconductor element having a sufficient critical current density can be produced by forming an intermediate layer of YSZ (yttrium-stabilized zirconia) which has well-oriented a-axis and b-axis on a polycrystalline substrate by means of a special process, and forming an oxide superconducting layer on the intermediate layer. With respect to this technology, the present inventors have filed applications by way of Japanese Patent Application No. Hei 4-293464, Japanese Patent Application No. Hei 8-214806, Japanese Patent Application No. Hei 8-272606, and Japanese Patent Application No. Hei 8-272607.
The technology proposed in these patent applications makes it possible to, when a film is formed on a polycrystalline substrate using a target made of YSZ, selectively remove YSZ crystals of unfavorable crystal orientation by means of an ion beam-assisted process in which the film forming surface of the polycrystalline substrate is irradiated in an oblique direction at an incident angle from 50 to 60xc2x0 with a beam of ions such as Ar+ thereby to selectively deposit YSZ crystals of good crystal orientation, so that an intermediate layer of YSZ crystal having good crystal orientation is formed.
According to the technology proposed in the previous applications of the present inventors, a polycrystalline thin film of YSZ with the a-axis and b-axis being favorably oriented can be formed. Also it was verified that the oxide superconducting layer formed on the polycrystalline thin film has a sufficient critical current density, and the inventors of the present application commenced research to develop a technology for producing polycrystalline thin films having more favorable properties than other materials.
FIG. 17 is a sectional view showing an example of the oxide superconductor element which the inventor shave been using recently. The oxide superconductor element D of this example has a four-layer structure made by forming, with the ion beam-assisted technology described previously, an orientation control intermediate layer 6 made of YSZ or MgO on a substrate 5 in the form of metal tape, then forming a reaction stopper intermediate layer 7 made of Y2O3 thereon and forming the oxide superconducting layer 8 thereon.
The reason for using the four-layer structure is that, in order to form an oxide superconducting layer having a composition of Y1Ba2Cu3O7xe2x88x92X, it is necessary to apply a heat treatment at a temperature in a range from 600 to 800xc2x0 C. after forming the oxide superconducting layer which has the desired composition by sputtering or other film forming process, but diffusion of elements may proceed between the oxide superconducting layers that have the compositions of Y1Ba2Cu3O7xe2x88x92x and YSZ, due to the heat supplied during the heat treatment, while the diffusion may cause deterioration of the superconducting property and must be prevented. The YSZ crystal which constitutes the orientation control intermediate layer 6 has a cubic crystal structure, and the oxide superconducting layer having the composition of Y1Ba2Cu3O7xe2x88x92x has a crystal structure called perovskite. Both of these crystal structures belong to a class of face-centered cubic crystals and have similar crystal lattices, but there exists a difference of about 5% in the lattice size between the two structures. For example, distance between nearest atoms, namely the distance between an atom located at a corner of the cubic lattice and an atom located at the center of the face of the cubic lattice is 3.63 xc3x85 (0.363 nm) in the case of YSZ, 3.75 xc3x85 (0.375 nm) in the case of Y2O3, and is 3.81 xc3x85 (0.381 nm) in the case of the oxide superconducting layer having the composition of Y1Ba2Cu3O7xe2x88x92X. Thus Y2O3 has an intermediate value between those of YSZ and Y1Ba2Cu3O7xe2x88x92X and is useful for bridging the difference in lattice size and can be advantageously used as a reaction stopper layer due to the similarity in the compositions.
With the four-layer structure shown in FIG. 17, however, the number of required layers is increased, which leads to a problem of increased number of production processes.
Through research to form a film of well-oriented crystal structure, made of a material which has a value of distance between nearest atoms nearer to that of the oxide superconductor than YSZ, directly on the metal tape substrate 5, the present inventors have completed the present invention.
Meanwhile techniques to form various films of good orientation on polycrystalline substrates have been used in fields other than the application of the oxide superconducting material, such as the optical thin film, magneto-optical disk, circuit wiring board, high-frequency waveguide, high-frequency filter and cavity resonator. In any of these fields, it remains a challenge to form a favorably oriented polycrystalline thin film of stable film quality on a substrate. A polycrystalline thin film having satisfactory crystal orientation would make it possible to improve the quality of an optical thin film, a magnetic film or thin film for circuit wiring to be formed thereon. Furthermore, it will be more preferable to be capable of forming the optical thin film, magnetic thin film, or thin film for circuit wiring, which has satisfactory crystal orientation, directly on the substrate.
The present invention has been made to solve the problems described above, and has been completed after intensively researching methods for forming a polycrystalline layer having favorable crystal orientation on a substrate by applying the ion beam assisted technology which the present inventors had previously proposed, and an object of the present invention is to obtain a polycrystalline thin film of composite oxide having good crystal orientation and distance between nearest atoms similar to the distance between nearest atoms of an oxide superconducting material formed on the film forming surface of a substrate.
Another object of the present invention is to provide a production method capable of forming such a polycrystalline thin film of good crystal orientation at high speed, and a method of producing a polycrystalline thin film of improved crystal orientation.
Still another object of the present invention is to provide an oxide superconductor element which has a high critical current density and high stability by forming an oxide superconducting layer on the polycrystalline thin film.
In order to achieve the objects described above, the polycrystalline thin film of the present invention is made of a composite oxide having a pyrochlore type crystalline structure of a composition represented as either AZrO or AHfO (A in the formula represents a rare earth element selected from among Y, Yb, Tm, Er, Ho, Dy, Eu, Gd, Sm, Nd, Pr, Ce and La) being formed on the film forming surface of a polycrystalline substrate, wherein the grain boundary inclination angles (the grain boundary misalignment angle), between the same crystal axes of different crystal grains in the polycrystalline thin film along the plane parallel to the film forming surface of the polycrystalline substrate, are controlled within 30xc2x0.
In the polycrystalline thin film of the composite oxide having a pyrochlore type crystalline structure of composition represented as either AZrO or AHfO, the rare earth element and Zr or Hf may be included in a relative proportion of 1:1.
The rare earth element and Zr or Hf which constitute the polycrystalline thin film of the composite oxide that is made mainly of a pyrochlore type crystalline structure of a composition represented as either AZrO or AHfO may be included therein in a relative proportion in a range from 0.1:0.9 to 0.9:0.1, and may be of a cubic crystal system. In this case, the crystal structure is not necessarily of a pyrochlore type, and may also have a similar structure called the omission fluorite type or the type C rare earth, which is valid as long as the cubic crystal system is maintained.
In the constitution described above, the polycrystalline substrate can be formed from a heat resistant metal tape made of a Ni alloy or the like, and the polycrystalline thin film can be formed from Sm2Zr2O7 or Gd2Zr2O7.
In the polycrystalline thin film of the constitution described above, the grain boundary inclination angles (the grain boundary misalignment angle), between the same crystal axes of different crystal grains in the polycrystalline thin film along the plane parallel to the film forming surface of the polycrystalline substrate, are preferably controlled within 200 and more preferably may be controlled within 100
In order to achieve the object described above, the present invention provides a method of producing the polycrystalline thin film comprising crystal grains of composite oxide having a pyrochlore type crystal structure of a composition represented by one of formulas AZrO and AHfO (A in the formula represents a rare earth element selected from among Y, Yb, Tm, Er, Ho, Dy, Eu, Gd, Sm, Nd, Pr, Ce and La) being formed on the film forming surface of a polycrystalline substrate, with the grain boundary inclination angles(the grain boundary misalignment angle) between the same crystal axes of different crystal grains along the plane parallel to the surface of the polycrystalline substrate whereon the film is to be formed are controlled within 30 degrees, wherein the polycrystalline substrate is heated to a temperature within 300xc2x0 C. and an ion beam of Ar+ ions, Kr+ ions, or Xe+ ions, or a mixed beam of these ions is generated from an ion source with the energy of the ion beam being set in a range from 150 eV to 300 eV, while the incident angle of the ion beam irradiating the substrate is set in a range from 50 to 60xc2x0 from the normal line of the film forming surface thereof, when depositing the particles generated from a target, which is made of the same elements as those of the polycrystalline thin film, on the polycrystalline substrate.
In the method of producing the polycrystalline thin film of the composition described above, the energy of the ion beam generated from the ion source is preferably controlled in a range from 175 eV to 225 eV, and more preferably at 200 eV when depositing the particles, generated from the target made of the same elements as those of the polycrystalline thin film, on the polycrystalline substrate.
In the method of producing the polycrystalline thin film of the composition described above, the ion beam irradiating the substrate is set in a range from 55 to 60xc2x0 from the normal line of the film forming surface thereof, and more preferably at 55xc2x0 from the normal line of the film forming surface when depositing the particles generated from a target, which is made of the same elements as those of the polycrystalline thin film, on the polycrystalline substrate.
Also in the method of producing the polycrystalline thin film of the composition described above, it is preferable that the polycrystalline substrate be heated to a temperature of 200xc2x0 C. and an ion beam of Ar+ ions, Kr+ ions, or Xe+ ions or a mixture of these ions is generated from the ion source with the energy of the ion beam being controlled to 200 eV, while the ion beam is applied to irradiate the substrate at an incident angle of 55xc2x0 from the normal line of the film forming surface thereof, when depositing the particles generated from the target, which is made of the same elements as those of the polycrystalline thin film, on the polycrystalline substrate.
In order to achieve the object described above, the present invention provides an oxide superconductor element comprising the polycrystalline substrate, the polycrystalline thin film, which is made of oxide crystal grains of composite oxide having a pyrochlore type crystal structure of composition represented by one of formulas AZrO and AHfO (A in the formula represents a rare earth element selected from among Y, Yb, Tm, Er, Ho, Dy, Eu, Gd, Sm, Nd, Pr, Ce and La) being formed on the film forming surface of the polycrystalline substrate with the grain boundary inclination angles(the grain boundary misalignment angle) between the same crystal axes of different crystal grains along the plane parallel to the surface of the polycrystalline substrate whereon the film is to be formed being controlled within 300 and an oxide superconducting layer formed on the polycrystalline thin film.
In the oxide superconductor element having the constitution described above, the oxide superconducting layer may also be made of an oxide superconducting material of a composition represented by one of formulas A1Ba2Cu3O7xe2x88x92X and A2Ba4Cu8OX, (A in the formula represents a rare earth element selected from among Y, Yb, Tm, Er, Ho, Dy, Eu, Gd, Sm, Nd, Pr, Ce and La), or a superconducting material of a cubic crystal system having another composition.
In the oxide superconductor element having the constitution described above, a heat resistant metal tape can be used as the polycrystalline substrate.
In the oxide superconductor element having the constitution described above, the grain boundary inclination angles(the grain boundary misalignment angle), between the same crystal axes of different crystal grains in the oxide superconducting layer along the plane parallel to the film forming surface of the polycrystalline substrate, may be controlled within 30xc2x0.
In order to achieve the object described above, the present invention provides a method of producing an oxide superconductor element comprising the polycrystalline substrate, the polycrystalline thin film made up of crystal grains of composite oxide having pyrochlore type crystal structure of composition represented by one of formulas AZrO and AHfO (A in the formula represents a rare earth element selected from among Y, Yb, Tm, Er, Ho, Dy, Eu, Gd, Sm, Nd, Pr, Ce and La) being formed on the film forming surface of the polycrystalline substrate, with the grain boundary inclination angles (the grain boundary misalignment angle) between the same crystal axes of different crystal grains along the plane parallel to the surface of the polycrystalline substrate whereon the film is to be formed are controlled within 30xc2x0, and the oxide superconducting layer formed on the polycrystalline thin film, wherein the polycrystalline substrate is heated to a temperature not higher than 300xc2x0 C. and an ion beam of Ar+ ions, Kr+ ions, or Xe+ ions or a mixture of these ions is generated from an ion source with the energy of the ion beam being set in a range from 150 eV to 300 eV, while the incident angle of the ion beam irradiating the substrate is set in a range from 50 to 60xc2x0 from the normal line of the film forming surface thereof, when depositing the particles generated from a target, which is made of the same elements as those of the polycrystalline thin film, on the polycrystalline substrate, and thereafter forming the oxide superconducting layer formed on the polycrystalline thin film.
The polycrystalline thin film of pyrochlore type crystal structure formed on the polycrystalline substrate is considered to be more advantageous than the conventional polycrystalline thin film of YSZ with many respects when a superconducting layer made of an oxide is formed thereon.
First, the lattice constant of ZrO2 which is the main component of the YSZ crystal is 5.14 xc3x85 (0.514 nm) and, assuming that the distance between an atom located at the center of a face of the face-centered cubic lattice of ZrO2 and an atom located at a corner of the face (distance between nearest atoms) in the face-centered cubic lattice of ZrO2 is 3.63 xc3x85 (0.363 nm), then the lattice constant of Sm2Zr2O7 crystal is 10.59 xc3x85 (1.059 nm) and the distance between nearest atoms is 3.74 xc3x85 (0.374 nm). Taking into account the fact that the distance between nearest atoms of oxide superconducting material having the composition of Y1Ba2Cu3O7xe2x88x92X is 3.81 xc3x85 (0.381 nm), the polycrystalline thin film made of Sm2Zr2O7 composite oxide is considered to be more advantageous with respect to the crystal matching than the polycrystalline thin film of YSZ. That is, when depositing the atoms of the polycrystalline thin film by the ion beam assisted process, normal deposit of atoms would be more easily achieved by using a material having a smaller value of distance between nearest atoms. Also, because Sm2Zr2O7 has the crystal structure of the pyrochlore type, a material having pyrochlore type crystal structure represented by one of formulas Gd2Zr2O7 (distance between nearest atoms 3.72 xc3x85 (0.372 nm)), La2Zr2O7 (distance between nearest atoms 3.81 xc3x85 (0.381 nm)), Ce2Zr2O7 (distance between nearest atoms 3.78 xc3x85 (0.378 nm)), Pr2Zr2O7 (distance between nearest atoms 3.78 xc3x85 (0.378 nm)), Gd2Hf2O7 (distance between nearest atoms 3.72 xc3x85 (0. 372 nm)), Sm2Hf2O7 (distance between nearest atoms 3.74 xc3x85 (0.374 nm)) and La2Hf2O7 (distance between nearest atoms 3.81 xc3x85 (0.381 nm)) may be used.
As other materials which have a pyrochlore type crystal structure, materials represented by formulas Y2Zr2O7, Yb2Zr2O7, Tm2Zr2O7, Er2Zr2O7, Ho2Zr2O7, Dy2Zr2O7, Eu2Zr2O7, Nd2Zr2O7, Y2Zr2O7, Y2Hf2O7, Yb2Hf2O7, Tm2Hf2O7, Er2Hf2O7, Ho2Hf2O7, DY2Hf2O7, EU2Hf2O7, Nd2Hf2O7, Pr2Hf2O7, and Ce2Hf2O7 may also be used.
The rare earth element and Zr or Hf which constitute the polycrystalline thin film of the pyrochlore type crystalline structure may be included therein in a relative proportion in a range from 0.1:0.9 to 0.9:0.1, instead of 1:1. In this case, the crystal structure is not necessarily of a pyrochlore type, audit may also have a similar structure called the omission fluorite type or the type C rare earth, which is valid as long as the cubic crystal system is maintained.
According to the present invention, the polycrystalline thin film comprising pyrochlore type crystal grains such as AZrO, AHfO or the like of good crystal orientation which is formed on the polycrystalline substrate with the grain boundary inclination angles(the grain boundary misalignment angle) being controlled within 30 degrees, and a polycrystalline thin film of pyrochlore type such as SmZrO having good crystal orientation is preferably used as a base layer for various thin films to be formed thereon, and makes it possible to achieve good superconducting property in the case in which the thin film to be formed is a superconducting layer, achieve good optical property in the case in which the thin film to be formed is an optical thin film, achieve good magnetic property in the case in which the thin film to be formed is a magnetic thin film, and obtain a thin film of lower wiring resistance and fewer defects in the case in which the thin film to be formed is used for circuit wiring.
As the other pyrochlore type composite oxide used for the polycrystalline thin film described above, a composite oxide represented by one of the formulas Gd2Zr2O7, La2Zr2O7, Ce2Zr2O7, Pr2Zr2O7, Gd2Hf2O7, Sm2Hf2O7 and La2Hf2O7 may be used, or a composite oxide represented by one of the formulas Y2Zr2O7, Yb2Zr2O7, Tm2Zr2O7, Er2Zr2O7, Ho2Zr2O7, Dy2Zr2O7, Eu2Zr2O7, Nd2Zr2O7, Y2Zr2O7, Y2Hf2O7, Yb2Hf2O7, Tm2Hf2O7, Er2Hf2O7, Ho2Hf2O7, Dy2Hf2O7, Eu2Hf2O7, Nd2Hf2O7, Pr2Hf2O7 and Ce2Hf2O7 may also be used.
A heat resistant metal tape made of an Ni alloy may be used as the polycrystalline substrate of the present invention, and a metal tape with the polycrystalline thin film comprising the pyrochlore type crystal grains formed thereon can be made.
According to the present invention, when particles of the target made of pyrochlore type composite oxide having a composition represented by one of formulas AZrO and AHfO (A in the formula represents a rare earth element selected from among Y, Yb, Tm, Er, Ho, Dy, Eu, Gd, Sm, Nd, Pr, Ce and La) are deposited on the polycrystalline substrate, the substrate is controlled to a temperature within 300xc2x0 C., the energy of the ion beam is set in a range from 150 eV to 300 eV, and the incident angle of the ion beam irradiating on the substrate is set in a range from 50 to 60xc2x0 from the normal direction of the film forming surface, and therefore it becomes possible to form the polycrystalline thin film of the pyrochlore type composite oxide with good crystal orientation with favorable grain boundary inclination angles (the grain boundary misalignment angle), which has been impossible in the prior art.
When the oxide superconducting layer is formed on the polycrystalline thin film of the pyrochlore type composite oxide having composition represented by one of formulas AZrO and AHfO (A in the formula represents a rare earth element selected from among Y, Yb, Tm, Er, Ho, Dy, Eu, Gd, Sm, Nd, Pr, Ce and La) which has good crystal orientation as described above, the oxide superconducting layer having good crystal orientation can be formed, and therefore, a oxide superconducting layer having a high critical current density and a high critical current can be made. This is because the polycrystalline thin film of the pyrochlore type composite oxide has better crystal matching characteristic with the oxide superconducting layer than the polycrystalline thin film of YSZ does, and this makes it possible to obtain the oxide superconducting layer having better crystal orientation than in the case of using the polycrystalline thin film of YSZ.
Moreover, the polycrystalline thin film of the pyrochlore type composite oxide of better crystal orientation can be made in a shorter period of time by the production method of the present invention than the YSZ polycrystalline thin film which the present inventors previously proposed.